An apparatus of this type is used recently mainly for applying a technique receiving a lot of attention and generally indicated by Polymerase Chain Reaction (PCR). This technique is based on a cyclically repeated DNA synthesis by means of a thermostable DNA polymerase. For applying this PCR technique the sample containers with the samples to be treated should be able to be maintained at three different temperatures during three different periods. Further passing through these temperatures should be cyclically repeated a number of times. As an example a cycle can be mentioned of subsequently 95.degree. C. during 1 minute, 37.degree. C. during 30 s and 65.degree. C. during 3 minutes, which cycle should be passed through 32 times.
In a known apparatus of this type the water bath is filled each time from one of three reservoirs in which water with the respective desired temperature is contained, in order to realize the three desired temperatures. Thereby the known apparatus has a rather complicated construction. Moreover the water in the different reservoirs has to be maintained at the desired temperature, wherein the water at the high temperature of 95.degree. C. evaporates quickly, while the water at the low temperature of 95.degree. C. evaporates quickly, while the water at the low temperature of 37.degree. C. forms an ideal medium for pollutants. Further this known apparatus has the disadvantage that the sample containers are heated indirectly by the water, whereby it takes a long time before the samples themselves have reached the desired temperature. The overall process thereby takes a lot of time.
In an other known apparatus Peltier elements are used for realizing the different desired temperatures. This known apparatus also has the disadvantage that the desired temperature of the samples themselves is reached only slowly due to the indirect heating. Further the temperature distribution along the carrier for the sample containers heated by the Peltier elements is not uniform.